1. Field of the Invention
The present invention relates to scanning probe microscopes used for the observation of a fine surface configuration of a sample.
2. Description of the Related Art
Well-known microscopes of this type include a scanning tunneling microscope (STM), atomic force microscope (AFM), magnetic force microscope (MFM), etc.
The STM, which is an apparatus proposed by Binnig or Rohrer et al. in 1982, can observe the surface configuration of an electrically conductive sample in atomic order. The details of this apparatus are described in "G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel; Surface Studies by Scanning Tunneling Microscope, Phys. Rev. Lett., Vol. 49 57 (1982)." The STM has an electrically conductive probe, which is supported in the vicinity of the conductive sample. When a voltage is applied between the probe and the sample after the tip end of the probe is brought to a position at a fine distance of about 1 nm from the sample surface, a tunnel electric current flows between the probe and the sample. This tunnel current changes depending on the distance between the probe and the sample, and its value changes substantially by a figure as the distance changes by 0.1 nm.
The probe is moved (e.g., for raster scanning) along the sample surface. During this movement, a control voltage is applied to a piezoelectric body so that the value of the tunnel electric current flowing between the probe and the sample is constant. By doing this, the distance between the probe and the sample can be kept fixed. Accordingly, the tip end of the probe is displaced following the surface configuration of the sample. Position data corresponding to this displacement of the probe end is calculated according to the control voltage applied to the piezoelectric body. A three-dimensional image of the sample surface is detected on the basis of this calculated data.
The AFM is proposed as an apparatus through which the surface configuration of an insulator can be observed in atomic order The details of this apparatus are described in "G. Binnig, C. F. Quate; Atomic Force Microscope, Phys. Rev. Lett., Vol. 56 930 (1986)."
In the AFM, a probe is supported by means of a flexible cantilever. When the probe is brought close to the surface of a sample, an attraction is produced between atoms of the probe end and those of the sample surface by a Van der Waals interaction. If the probe is brought closer to the sample surface so that the distance between them is substantially equal to the atomic distance or bond length, a repulsive force based on the Pauli exclusion principle acts between the atoms of the probe end and the sample surface. The attraction and the repulsive force (atomic force) are as small as 10.sup.-9 to 10.sup.-12 N. When the atoms of the probe end are subjected to the atomic force, the cantilever is displaced for a distance corresponding to the magnitude of the force. When the probe is moved along the sample surface to scan it, the distance between the probe and the sample changes depending on the irregularity of the sample surface, whereupon the cantilever is displaced. This displacement of the cantilever is detected, and the piezoelectric body or some other inching element is feedback-controlled to keep the cantilever's displacement constant. Since the voltage then applied to the piezoelectric body is changed corresponding to the surface configuration of the sample, an irregularity image of the sample surface is detected in accordance information on the applied voltage.
The MFM, which has a probe formed of a magnetic material, is basically constructed in the same manner as the AFM. Like the AFM, the MFM is designed so that an irregularity image of the surface of a sample is detected by scanning the sample surface by means of the probe in a manner such that a magnetic force acting between the probe and magnetic particles is kept constant.
The AFM or MFM can be also used as an STM if the probe on the cantilever used therein is formed of an electrically conductive material such that the tunnel electric current can be detected thereby.
Referring now to FIGS. 13 and 14, the cantilever used in each of the aforementioned microscopes will be described. In these drawings, numeral 1 denotes the cantilever which is made of SiO.sub.2 (or Si.sub.3 N.sub.4) and is rectangular in external shape.
As shown in FIG. 13, a fixed end portion (hereinafter referred to as base portion b) of the cantilever 1 is mounted on a Pyrex member 2. A probe 3 protrudes downward for a fine distance from the underside of the free end portion of the cantilever 1, on the opposite side thereof to the base portion b. The probe 3 is formed in the following manner.
First, SiO.sub.2 is deposited on the cantilever 1. Then, an opening mask is disposed over the deposited surface of the cantilever, at a distance substantially equal to the length of the probe therefrom. Then, a deposition material is built up on the cantilever 1 from above the opening mask so that it forms a cone tapered toward the center. In order to obtain a large displacement against a very small force, such as an atomic or magnetic force, moreover, the cantilever 1 is formed of a thin plate of a material which is as light in weight and has a modulus of elasticity as high as possible.
If an external force F is applied to a free end portion of a cantilever spring which has a thickness a, base width b, and length l, a displacement .epsilon..sub.V of the free end portion with respect to a Z direction (see FIGS. 13 and 14) is given by EQU .epsilon..sub.V =4l.sup.3 F/.sup.3 bE, (1)
and a Y-direction .epsilon..sub.H is given by EQU .epsilon..sub.N =4l.sup.3 F/ab.sup.3 E, (2)
where E is the modulus of longitudinal elasticity.
In equations (1) and (2), the values .epsilon..sub.V and .epsilon..sub.H can be made larger by increasing l and reducing a and b.
If the cantilever 1 is lengthened in this manner, its effective mass increases, and its natural frequency lowers, so that its follow-up performance is lowered when it is used to scan the sample surface.
The natural frequency f.sub.0 of the elastic material may be calculated according to EQU f.sub.0 =(E/m.sub.0).sup.1/2 /2.pi., (3)
where m.sub.0 is the load (i.e., effective mass) of the elastic material.
As seen from equation (3), the effective mass of the cantilever 1 can be reduced by forming its whole body into a thin, narrow rectangular structure and reducing the size of the probe 3 at the free end portion of the cantilever. As a result, the natural frequency of the cantilever increases, and the follow-up performance is improved.
Usually, the thickness (a), base width (b), length (l) of the cantilever are 0.3 or 0.6 .mu.m, 80 or 120 .mu.m, and 100 or 200 .mu.m, respectively. The length of the probe is about 2 .mu.m.
If the probe 3 is too short, however, the face of the cantilever 1 and the lower surface of the Pyrex member 2 are brought as close as about 2 .mu.m to the sample when the tip end of the probe approaches the sample to a distance of about 0.1 to 10 nm therefrom. As a result, the cantilever 1 may possibly run against the sample to be observed when the sample surface is irregular, in particular.
Accordingly, the probe 3 must have a sufficient length, in order to measure the sample surface with a long enough distance kept between the sample and the cantilever 1 lest the cantilever run against the sample.
If the probe 3 is too long, however, the mass of the cantilever 1 inevitably increases. When the probe 3, thus long, is moved along the sample surface to scan it, moreover, the position of the tip end of the probe 3 is bound to be shifted toward the sample surface (e.g., in the direction of arrow R of FIG. 14) by an intermolecular attraction from the sample, acting on the probe end, or an atomic force, such as a Van der Waals force. Such an awkward situation causes errors in measurement results.
A novel method has been proposed to solve this problem. According to this method, the base width (b) of the cantilever 1 is increased to reduce the amount of torsional rotation in the R direction (see FIG. 14).
The relationship between a turning moment (T), which is caused when a shearing force acts on the free end portion of the cantilever 1 through the probe 3, and a torsional angle .theta. (see FIG. 14) of the free end portion of the cantilever may be expressed as follows: EQU .theta.=3lT/a.sup.3 bG, (4)
where G is the modulus of transverse elasticity.
In connection with this, a shift amount .DELTA.M of the probe end is calculated according to EQU .DELTA.M=d sin .theta.. (5)
Even though the base width (b) is increased as aforesaid, however, the influence of the shearing force cannot be thoroughly removed. Since the increase of the base width (b) entails the increase of the mass of the cantilever 1, moreover, the natural frequency of the cantilever and its follow-up performance for the irregularity of the sample surface lower.
In the microscopes described above, the following two types of optical sensing systems are conventionally used as means for detecting the displacement of the cantilever 1.
In a first sensing system, an optical reflective surface is provided at the free end portion of the cantilever 1, and a ruby solid laser beam or an argon gas laser beam is applied to the reflective surface through a fiber. A change of the angle of reflection of the cantilever is detected by detecting the light beam reflected by the reflective surface, by means of an optical position sensor.
In a second sensing system, a laser beam emitted from a light source similar to the one used in the first sensing system is divided in two, a reference light beam and an incident light beam on the optical reflective surface. The reference light beam is caused to interfere with the reflected light beam from the reflective surface, and the displacement of the cantilever is detected by photoelectrically converting the resulting coherent output.
In these displacement sensing systems, however, the light beam applied to the cantilever is fully narrowed down, so that a large number of relatively large-sized optical elements are needed. As a result, the sensing system is large-sized and complicated in construction.
Since the alignment of an optical system has a great influence on the measurement accuracy, moreover, fine readjustment for the optical system alignment, which is required when the probe or cantilever is replaced with a new one, is a highly delicate job.